A Bostonian’s Guide to Smog and Stroke

The Air Pollution-Stroke Link and What to Do About It

Any list of risk factors for stroke typically includes diabetes, high cholesterol, high blood pressure, excess weight, physical inactivity, smoking and poor diet. Exposure to air pollution doesn't even make the list, unless you count smoking.

Yet a new study recently showed that even moderate levels of smog increase the risk of stroke by a dramatic 34 percent. Data that led to this conclusion was collected right here in the Boston area. The research was funded by the National Institute of Environmental Health Science and the U.S. Environmental Protection Agency and published in the Archives of Internal Medicine.

What is the significance of your findings for the average person?

The level of air pollution that causes a 34 percent increased risk is considered safe under federal regulations - but it is not safe. Particles in the air, mostly from vehicle traffic, were associated with a significantly higher risk of ischemic stroke (from a blood clot in the brain) on days when the EPA's air quality index for particulate matter was yellow instead of green. The data showed that the time between exposure to elevated pollution and the onset of stroke is less than a day.

How did you conduct the study?

We analyzed the medical records of more than 1,700 patients in the Boston area who went to the hospital for treatment of confirmed strokes between 1999 and 2008. We matched the onset of stroke symptoms in each patient to hourly measurements of particulate air pollution taken at the Harvard School of Public Health's environmental monitoring station, which is located on the roof of the Harvard Medical School Library in the Longwood Medical Area of Boston. For 90 percent of the patients, we were able to measure pollution within 13 miles of their homes. We looked at strokes that had been confirmed by attending neurologists and estimated the exact hour the strokes occurred, rather than the time they arrived at the Emergency Room. For all these reasons, we think the data are exceptionally precise.

Can you tell us more about the exact type of pollution that seems to cause strokes?

We looked at a type of particle the EPA calls "PM2.5." This means the particles have a diameter of 2.5 millionths of a meter. They come from a variety of sources, including power plants, factories, trucks, automobiles and burning wood. Black carbon and nitrogen dioxide from car and truck exhaust are especially linked with stroke.

How do the particles cause stroke?

The particles travel deeply into the lungs and trigger activation of irritant receptors in the lungs that send a signal to the brain, which results in increases in heart rate and blood pressure and makes the blood vessels stiffer. The air pollution also can cause inflammation in the lung that can spill over into the blood stream and heighten the tendency of the blood to form clots. The peak risk to patients from pollution exposure occurs 12 to 14 hours before a stroke.

Can you tell us more about air pollution in Boston?

Boston has relatively clean air compared to other parts of the country like Los Angeles, Chicago or Houston. Yet we still find that the risk of stroke is higher within moderate levels on days with more particles in the air. The type of air pollution that we studied is relatively uniform across the greater Boston area and our study included people living anywhere inside I-495.

Why do levels of air pollution vary from day to day in the Boston area? Is it a seasonal problem?

Air pollution levels vary from day to day and also from season to season. Elevated levels can happen at any time of the year, but tend to occur more often in the summer. The specific amount of air pollution and the mixture of pollutants are influenced by many factors. For example, while much of the pollution is generated locally from cars, trucks and other commercial vehicles, there is also a component from burning fuel to heat homes and businesses and from some local industries and power plants. In addition, some of the pollution that we are exposed to is transported, sometimes for hundreds of miles, from other regions of the country. The composition of the air pollution on any given day is also altered by weather patterns including air trajectories that influence where our local weather comes from.

What can an individual Bostonian do to protect him or herself from stroke caused by pollution?

The absolute risk of having a stroke triggered by air pollution is related to how high your risk is even in the absence of air pollution. Because of this, the best way to prevent having a stroke triggered by air pollution is to keep your risk low at all times. This means controlling your risk factors. For smokers, the single most important thing you can do to lower your risk is to stop smoking. Other important risk factors include getting your blood pressure and cholesterol checked and keeping them well-controlled. It is important for people with diabetes to keep it under good control, too. Regular exercise and a balanced, heart-healthy diet also help to keep the risk low.

People should follow the weather reports especially carefully in the warm weather months to be sure that if they have risk factors, they are not putting themselves at additional risk with a high level of activity.

The EPA also recommends when the Air Quality Index is in the "unhealthy" range that "People with heart or lung disease, older adults, and children should avoid prolonged or heavy exertion. Everyone else should reduce prolonged or heavy exertion."

The action recommendations from the EPA for reducing your exposure to particles are:

Planning strenuous activity when particle levels are forecast to be lower

Reducing the amount of time spent at vigorous activity

Choosing a less strenuous activity (e.g., going for a walk instead of a jog) on highly polluted days

What is the answer to this problem?

The answer is not easy - it's reducing pollution. The EPA is very responsive to new research and takes it into consideration each time the national air quality standards are set. As a result, air quality in the U.S. has improved over the past several decades, but we still have more work to do to further reduce the health consequences of air pollution. We estimate that reducing PM2.5 pollution by about 20 percent could have prevented 6,100 of the 184,000 hospitalizations for stroke in 2007.